There are four primary areas of assessment for this Project on the Neuropsychiatric Features of the Chronic Fatigue Syndrome: comprehensive evaluation and monitoring of psychiatric symptoms in the patient and his/her family; assessment of neuropsychological functioning, with heavy emphasis on higher-order functions; surveillance of functional status and perceptions of illness and well-being; and assessment of pain experience, threshold, and tolerance. We have a carefully chosen battery of tests, many of which we have tested in our previous studies of CFS. These measures will be compared within subjects across times of evaluation, cross-sectionally compared across different subject groups, and correlated across different Projects (e.g., with measures of autonomic functioning from the Neurophysiologic Project). We seek to understand what distinguishes CFS from other conditions with some shared symptoms (affective disorder, rheumatoid arthritis, allergy). These findings will yield a more comprehensive model of the neuropsychiatric processes specific to CFS patients. These results may inform future treatment strategies.